Introduction
Human consciousness flows continuously and spontaneously from one
thought to the next. Sometimes thinking evolves into an endless stream
of repetitive self-referential negative thoughts that are difficult to
let go of (Tseng & Poppenk, 2020), defined as rumination. According to
the Response Styles Theory, rumination is the focused attention on the
symptoms of one’s emotional distress and its possible causes and
consequences, as opposed to its solutions (Nolen-Hoeksema, 1991;
Nolen-Hoeksema et al., 2008). The habitual tendency to ruminate is
considered a maladaptive form of emotional regulation and has been
associated with a sustained physiological stress response that damages
the hormonal, cardiovascular, and nervous systems, ultimately leading to
observable physical and mental health problems (Aldao et al., 2010;
Brosschot et al., 2006; Kubzansky et al., 1997). Rumination is one of
the reasons responsible for incomplete cardiovascular recovery after
stress exposure and is therefore associated with cardiovascular indices
(Radstaak et al., 2011). Besides, even though the tendency to ruminate
is a well-known predictor of the onset and maintenance of depression
(McLaughlin et al., 2007; Nolen-Hoeksema et al., 2008), rumination is
considered a more general transdiagnostic factor of vulnerability and
outcome in stress-related psychopathology (e.g., alcohol abuse, eating
disorder, anxiety; Caselli et al., 2010; Dondzilo et al., 2016;
Wilkinson et al., 2013). Therefore, core neurophysiological mechanisms
underlying a tendency to ruminate are important to further understand
the relationship between rumination and the development of future
clinical symptoms and health problems.
The functional state of the Autonomic Nervous System (ANS), consisting
of the sympathetic and parasympathetic systems, has been reported as an
important neurophysiological mechanism associated with rumination
(Ottaviani et al., 2016; Vanderhasselt & Ottaviani, 2021). More
specifically, both branches of the ANS act on the sino-atrial node of
the heart and result in a complex variation between consecutive
heartbeats over time, defined as heart rate variability (HRV; Goldberger
et al., 2001; Porges, 2001, 2007; Saul et al., 1990; Task Force, 1996).
In the face of stressors, the inhibitory control of the parasympathetic
(over the sympathetic) nervous system enables the regulation of
physiological and psychological states, an ability that is considered
key to behavioral adaptability to the environment (mainly via the vagus
nerve; Thayer et al., 2012). This parasympathetic dominance is
associated with higher HRV (Friedman, 2007; Grol & De Raedt, 2020;
Nasso et al., 2019, 2020; Pulopulos et al., 2018). Higher HRV is
regarded as an indicator of better emotional regulation and mental
health (Brosschot et al., 2006; Perna et al., 2020; Reynard et al.,
2011; Thayer & Lane, 2000). In addition, the heart and the brain are
reciprocally connected via the ANS pathways. Thayer’s neurovisceral
integration model proposes a network of cortical and subcortical neural
structures, known as the central autonomic network (CAN), which receives
sensory input from the peripheral end organs such as the heart (Thayer
& Lane, 2000). The neurovisceral integration model proposes that
vagally-mediated HRV is a read-out of the bi-directional interaction
between central and cardiovascular processes, with higher HRV denoting
more self-regulation and adaptability (Thayer et al., 2009).
Even though the association between rumination and vagally-mediated HRV
has been widely studied, research remains inconsistent across the
literature. Rumination can be separated into trait and state components,
with the habitual tendency to ruminate in daily life referring to trait
rumination, whereas state rumination is the act of ruminating elicited
by discrete stressors (Key et al., 2008; Nolen-Hoeksema & Morrow,
1993). Most often, dynamic changes in HRV have been reported to examine
the link between state rumination and stress reactivity after a
laboratory induction (Ottaviani & Shapiro, 2011). In these studies,
where individuals were put into a state of rumination in the laboratory
- such as a video clip triggering stress, coping with fear sources that
frighten the individual, or engaging in fearful or angry imagery or
recall - a reduction of HRV was observed (Beauchaine et al., 2007;
Castaneda & Segerstrom, 2004; El-Sheikh et al., 2011). Furthermore, in
a meta-analysis reviewing 18 experimental studies, it was reported that
perseverative cognition, including rumination and worry that share a
commonality and are highly intercorrelated (Brosschot et al., 2006b;
Fresco et al., 2002), was associated with lower HRV in healthy
volunteers only on the state measures (Ottaviani et al., 2016). However,
in contrast to the inverse association between stress-induced HRV and
rumination response, questions remain regarding the association between
resting (baseline) HRV and the tendency to ruminate in daily life (trait
rumination). The habitual tendency to ruminate is known to be associated
with chronic stress (inability to recover) and has been associated with
a higher risk of cardiovascular disease (Busch et al., 2017; Schwartz et
al., 2003). Importantly, the meta-analysis mentioned above reported that
there was no association between HRV and the trait measure of
perseverative cognition in the experimental study; however, the number
of correlational studies (and the number of subjects) testing this
association was relatively low (Ottaviani et al., 2016). Therefore, the
relationship between resting HRV and trait rumination in healthy
individuals remains unclear, as many studies focus on clinical
populations or the HRV in response to induced rumination (i.e., a
momentary, reactive state). Even though resting HRV is assumed to be a
transdiagnostic and promising biomarker of mental health resilience
(Beauchaine & Thayer, 2015; Perna et al., 2020), to the best of our
knowledge, there is no study describing the association between
resting-state HRV and trait rumination based on a large-scale and
well-powered dataset in healthy individuals.
In this study, we use a large cross-sectional dataset of physiological
and self-report (trait rumination and depression) data from our
laboratory collected over a time span of 4 years (2017-2022) to
investigate the association between resting HRV (i.e., measured during a
baseline period) and the habitual tendency to ruminate in healthy
individuals. We hypothesize that baseline HRV at rest will be negatively
correlated with trait rumination as both high trait rumination and low
HRV are risk factors for stress-related psychopathology (Key et al.,
2008; Larsen & Christenfeld, 2009). In addition, given that HRV is
sensitive to various demographic variables, we will investigate whether
HRV is associated with the tendency to ruminate after controlling for
gender, age, and the level of depressive symptoms that are known to
significantly influence HRV (Laborde et al., 2017). Moreover, a study
has indicated that parasympathetic regulation of the heart through the
vagus nerve might not have an unmitigated linear relationship with
emotion (Kogan et al., 2013), so we also explore whether there is a
non-linear relationship between HRV and emotion in our large
cross-sectional data.